1. Field of the Invention
The present invention pertains to the discharging of capacitors used with defibrillators, and in particular to the automatic control of such discharge.
2. Description of the Related Art
Cardiotherapeutic defibrillators, once used only by trained medical personnel, are now being made available for use by the general population, including individuals having little or no training. The defibrillators contemplated for general use are of the automatic external type and include on-board real time diagnostic capability to intervene or otherwise control the defibrillator therapy being administered. In general, the defibrillators deliver a relatively high voltage, low energy pulse or series of pulses to a patient suffering cardiac arrhythmias, such as ventricular fibrillation. The power supply relied upon to deliver the defibrillation therapy typically comprises one or more batteries carried on board the defibrillator unit or an electrical power utility supplying mains power to a building, for example. Because of the nature of the electrical therapy required, it is not possible in a practical device to supply the therapeutic energy upon instantaneous demand, by drawing from the power source. Instead, energy from the power source must be accumulated over a certain period of time in one or more defibrillator capacitors which are later discharged to deliver the desired defibrillation therapy. It is particularly critical that the defibrillation therapy be delivered as quickly as possible, given the nature of the medical threat encountered. Accordingly, rapid charging of the defibrillator storage capacitor is required and advances in reducing charge time are still being sought.
Due to the nature of the use to which the defibrillation equipment is put, certain components employed must be carefully constructed to close performance tolerances which are expected to be closely maintained throughout the life of the component. It is important that such components are not unexpectedly stressed during unusual operating conditions, as when main power supply voltage unexpectedly drops. Also, it would be advantageous if a closer control could be exercised over the stress to which the electrical components are put.
In addition to rapid charging, practical defibrillation equipment must also be capable of rapid discharging in order to prepare for a controlled sequence of operation. Discharging may be required, for example, when a portable defibrillation unit is to be packed away for return transport to a hospital or dispatch office. At other times, discharging of defibrillator capacitor bank is required when the therapeutic action is requested to be performed at a lower capacitor voltage. For example, patients of different ages require adjustments in the defibrillation voltage applied. A patient's age may, for example, be indirectly conveyed to the defibrillation equipment by the choice of defibrillator paddles connected to the defibrillation equipment. A sophisticated, automated defibrillation unit could be informed of the paddle size and, accordingly, determine the defibrillation voltage needed, or otherwise could prompt an operator to confirm data indicating the defibrillation voltage required. In other types of defibrillation equipment in common use today, an operator is required to manually select the defibrillation voltage, either directly or indirectly through settings bearing various legends. An inexperienced or untrained field operator could, by cycling the defibrillator voltage setting, cause the voltage reduction circuit undue stress. Typically, the greatest stress is borne by a discharge resistor or the like dissipative disarmed device which can become extremely warm during this type of unusual operating condition. Unusually heavy use, even though otherwise, proper, could also cause unacceptable stress on a defibrillator disarm circuit.